Plasma arc torches typically include a nozzle for directing a process fluid at a workpiece and an electrode capable of supporting an electric arc such that the arc extends through the nozzle and attaches to the workpiece. Two general types of plasma arc torches are in common use, the gas-shielded torch and the water-injection torch. In a gas-shielded torch, a primary or plasma gas is directed through a plasma nozzle such that the plasma gas envelops and immediately surrounds the electric arc, and a secondary or shield gas is directed through a shield nozzle such that the shield gas surrounds the stream of plasma gas and the arc. The function of the plasma gas is to improve plasma generation and facilitate faster and more efficient cutting of the workpiece, while the function of the shield gas is to control the cutting process. In a water-injection torch, the work operation is controlled by directing water through a secondary or water-injection nozzle such that a jet of water surrounds the stream of plasma gas and the arc. The plasma and shield gases and the injection water are collectively referred to herein as process fluids.
Various process fluids are used in gas-shielded and water-injection torches, including nitrogen, oxygen, hydrogen, air, argon/hydrogen mixtures, methane, deionized water, and others. The type of process fluid used is typically selected based primarily on the material and thickness of the workpiece. For example, when cutting stainless steel with a gas-shielded torch, nitrogen or air is commonly used as the plasma gas and nitrogen mixed with methane or with an argon/hydrogen mixture is frequently used as the shield gas. However, when cutting carbon steel, oxygen is commonly used as the plasma gas and nitrogen or nitrogen mixed with oxygen is typically used as the shield gas.
When a plasma arc torch is to be used first for cutting a workpiece requiring one type of process fluid, and then for cutting a different workpiece requiring another type of process fluid, it is generally necessary to purge the first process fluid from the torch passages and the supply line which supplies the process fluid to the torch, before introducing the second type of process fluid into the supply line and torch passages. This is particularly true where the two successively used process fluids are reactive with each other, such as oxygen and hydrogen, inasmuch as mixing of these fluids within the supply line or torch could be extremely hazardous. Accordingly, following completion of a first work operation using a first process fluid, an inert purge gas, typically nitrogen, is usually supplied through the supply line for a period of time sufficient to purge substantially all of the first process fluid from the supply line and from the torch passages and nozzle. The second process fluid for the new work operation is then supplied through the supply line, and is normally allowed to flow for a period of time sufficient to displace the purge gas and fill the supply line and the torch passages with the second process fluid.
In plasma arc torch systems which are currently commercially available, the operator of the plasma arc torch machine must manually set switches or otherwise act so that the appropriate valves are opened and closed for purging the supply line and torch of an old process fluid and filling the supply line and torch with a new process fluid. The operator typically consults a chart or the like and looks up a new process fluid requirement for a new workpiece based on the identity of the workpiece or the material type and thickness of the workpiece. Accordingly, the process of purging and filling is subject to error. For example, the operator may misread the chart, or may read the chart correctly but operate the valves incorrectly, so that the wrong process fluid is selected and used in the new process. The result frequently is an unsatisfactory work operation, causing the workpiece to have to be scrapped.
A further problem is that the operator may forget to purge the old process fluid from the lines and passages before switching to the new process fluid and starting a new work operation, or may purge for too short a time period, with the result that two different process fluids mix within the lines and passages. If the two different process fluids are reactive with each other, the result can be extremely hazardous.
Additionally, when both purging and filling, the operator may allow the purge gas or new process fluid to flow for a longer period of time than necessary to adequately displace the existing gas in the supply line and torch passages. This may result from either inattentiveness or an abundance of caution by the operator, but in either case both time and fluids can be wasted.